Low density silica particles and method for their preparation

ABSTRACT

Internal porosity is created in dense SiO 2  particles by heating a mixture of the particles with B 2  O 3  at a temperature above about 450° C. Resulting SiO 2  particles have altered morphology, internal porosity, and small to negative volume changes when heated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.09/092,159 filed Jun. 5, 1998, now abandoned, which is acontinuation-in-part of application Ser. No. 08/871,162 filed Jun. 9,1997, now U.S. Pat. No. 5,861,134.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to a process for preparing low density silicaparticles by heating a mixture of boric acid or B₂ O₃ with high densitysilica particles at a temperature above about 450° C. Silica particlesso prepared individually may have internal porosity and exhibit uniqueproperties, such as a small to negative thermal expansion coefficientwhen first heated after preparation. The particles are useful for avariety of applications, such as a catalyst support or in resin blendsused in electronic packaging.

2. Background Information

Silica particles are well known in the art and have found a broad rangeof applications due to the chemical inertness of silica and itsfavorable dielectric properties. Common applications include use as acatalyst support, as media in liquid chromatography, and as filler inresins employed in electronic packaging.

Silica generally is prepared using a mechanical, fumed, or chemicalprocess. Conventional mechanical processes produce solid particleshaving little or no internal porosity. Variations in bulk densities areattributed to particle size and how tightly the particles are packed.Conventional fumed silicas are produced by high temperature (1800° C.)hydrolysis of silicon tetrachloride. This process is expensive, producescorrosive hydrogen chloride as a by-product, and the product silica hasan undesirable low bulk density. Conventional chemical processes forproducing xerogels and aerogels do create particles having internalporosity, but these processes are even more expensive than those forfumed silicas, and there is limited commercial capacity. The lower costmechanical processes tend to produce particles having an angularmorphology that are undesirable for applications such as resin/fillersystems used in electronic packaging.

Thus, there continues to be a need for economical methods of producingsilica particles having internal porosity. Also, there continues to be aneed for silica particles having unique properties.

SUMMARY OF THE INVENTION

It now has been found that internal porosity can be introduced to silicaparticles, and any surface angularity can be reduced, by heating thesilica particles in the presence of B₂ O₃ to a temperature above about450° C. to a temperature below the liquidus curve for the mixture.Accordingly, the present invention provides a process for preparingsilicon dioxide particles comprising, in sequence:

a. forming a mixture of dense naturally occurring or fused silicondioxide particles having a surface area less than 5 m² /g and at leastone boron compound selected from the group consisting of B₂ O₃ and boricacid;

b. heating said mixture to a temperature in the range of 450° C. to thetemperature at the liquidus curve of said mixture for a time sufficientto alter the density and/or morphology of said individual silicondioxide particles;

c. cooling the mixture to ambient conditions; and

d. recovering discrete silicon dioxide particles, wherein the recoveredsilicon dioxide particles have an altered density and/or morphology ascompared to the silicon particles of step a.

It also has been discovered that the recovered silicon dioxide particleshave unusual properties. For those recovered silicon dioxide particlesheated to a temperature above about 450° C. to about the temperature atthe liquidus curve of the mixture, the particles have the property ofhaving a low average volume change of about ±0.5% over the temperaturerange of 100-900° C. This property makes the particles particularlyuseful as filler to adjust the thermal expansion coefficient of resinsapplied during the fabrication of articles, such as electronicpackaging, to minimize thermal induced stresses during the manufactureprocess. Accordingly, this invention also provides discrete porous SiO₂particles that have a low average volume change of about ±0.5% over thetemperature range of 100-900° C.

For those particles heated to a temperature above the liquidus curve ofthe mixture, the particles have the property of shrinking whensubsequently heated to a temperature in the range of approximately 100°to 400° C. Accordingly, this invention also provides discrete porousSiO₂ particles that have an Initial Negative Expansion Coefficient, asthe term is defined herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a phase diagram for crystalline silica and B₂ O₃.

FIG. 2 illustrates dimensional changes observed in pellets made from alot of particles prepared in accordance with the invention by heatingsilica particles in the presence of B₂ O₃ to a temperature above theliquidus curve, upon a subsequent heating.

FIG. 3 illustrates dimensional changes observed in pellets made from asecond lot of particles prepared in accordance with the invention byheating silica particles in the presence of B₂ O₃ to a temperature abovethe liquidus curve, upon a subsequent heating.

FIGS. 4 through 7 illustrate dimensional changes observed in pelletsmade from commercial lots of particles upon heating.

FIG. 8 illustrates dimensional changes observed in pellets made from asecond lot of particles prepared in accordance with the invention byheating silica particles in the presence of B₂ O₃ to a temperature ofabout 500° C., upon a subsequent heating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The silica particles of the invention conveniently are prepared byforming a mixture of B₂ O₃ or boric acid (H₃ BO₃) with amorphous orcrystalline silica particles having a surface area less than 5 m² /g andheating the mixture to a temperature above about 450° C. as described indetail hereinbelow. The mixture then is cooled and the B₂ O₃ is removed.Silica particles thereby formed have internal porosity.

Silica/Boron Mixture

The selected silica particles are dense amorphous or crystallinematerials, having a specific surface less than 5 m² /g, and typicallyhave an average particle size in the range of 5 to 500 microns althoughadvantages of the invention may be obtained with larger or smallerparticles. Expensive specially manufactured silica particles having lowdensity (such as silica gels, xerogels, and aerosils), having a specificsurface area greater than 5 m² /g, typically above 200 m² /g, are notselected in practicing the invention. Fused silica particles may beselected, but such materials are expensive and offer no advantage.Typical starting materials include naturally occurring materials such asbeach sand and quartz. These materials have a pore volume less than 0.1cc/g and a bulk density greater than 0.9 g/cc.

As used herein, surface area (m² /g) is determined in accordance withthe BET method described by S. Brunauer, P. H. Emmett, and E. Teller inJournal American Chemical Society, Vol. 60, p. 309 (1938). The surfacearea is calculated using the adsorption of nitrogen at its boilingpoint. The amount of adsorbed nitrogen is measured using a MicromeriticsASAP 2400 instrument. The BET equation calculates the amount of nitrogencorresponding to a monolayer. Surface area is calculated using an areaof 16.2 square angstroms per nitrogen molecule.

The silica particles are blended with B₂ O₃, boric acid, or otherprecursors that forms B₂ O₃ in situ, in dry form or preferably with theaddition of water to facilitate formation of an intimate mixture. Asused herein, the compound "B₂ O₃ " includes a precursor compound thatbreaks down in situ to form B₂ O₃. Conventional mixing equipment isemployed for this purpose.

The effect of B₂ O₃ on the silica particles may be observed at lowlevels, such as with mixtures containing 5% B₂ O₃ by weight. Typicallythe mixture will contain 30% to 70% B₂ O₃, preferably 30% to 60% B₂ O₃,wherein the percentages are weight percentages of the total SiO₂ and B₂O₃ mixture. If boric acid or another precursor of B₂ O₃ is selected, thequantity added is adjusted such that the actual quantity of B₂ O₃ iswithin the desired range.

Selection of the optimum amount of B₂ O₃ varies with the SiO₂ particlesselected for the process, heating "soak time" as discussed hereinafter,and the extent to which it is desired to introduce internal porosity tothe particles. Another factor taken into account is the cost ofprocessing and recovering B₂ O₃ in the process.

Heating Step

The blend of SiO₂, B₂ O₃ and/or boric acid, and water (if introduced asa mixing aid) is heated to an elevated temperature. Any boric acidpresent decomposes to form B₂ O₃ and water, which rapidly evolves and isremoved from the mixture along with any water present as a mixing aid.

As illustrated in FIG. 1, B₂ O₃ melts at approximately 450° C. at allconcentrations. The temperature at which crystalline SiO₂ (e.g., quartzand related tridymite and cristobalite) also melts, however, varies withthe relative proportions of SiO₂ and B₂ O₃ contained in the mixture, asreported by T. J. Rockett and W. R. Foster in Journal of AmericanCeramic Society, 48 [2], pages 75 to 80 (1965). The phase diagram foramorphous SiO₂ and B₂ O₃ is similar, with the amorphous silica meltingat a temperature between about 450° C. and 1600° C. that varies with therelative proportions of SiO₂ and B₂ O₃ contained in the mixture. Theterm "liquidus curve" refers to the plot of composition-dependenttemperatures at which the SiO₂ and B₂ O₃ are both in the liquid phase.

In accordance with the invention, the mixture is heated to a temperatureabove about 450° C. It will be appreciated that the SiO₂ may not melt atthis temperature, but there is a "soak time" during which the elevatedtemperature takes effect. The desired soak time will vary with theselected temperature, size and morphology of the SiO₂ particles, and theextent to which it is desired to introduce internal porosity in the SiO₂particles. Not much effect is observed at soak times as short as 30minutes when a temperature is selected only slightly above 450° C. Theselected soak time typically will be 1 to 20 hours, preferably 1 to 4hours.

The impact on the silica particles of treatment with B₂ O₃ begins atabout 450° C. and becomes more pronounced at higher temperatures belowthe liquidus curve. Depending on the desired properties in the productsilicon dioxide particles, the B₂ O₃ /SiO₂ mixture may be heated totemperatures closer to the liquidus curve for the particular mixturethat has been selected.

When the mixture is heated to temperatures above the liquidus curve,there is a similar "soak time" during which SiO₂ particles becomeliquid. It is not desirable to totally melt the SiO₂ particles. Theselected soak time for temperatures above the liquidus curve is alsotypically 1 to 20 hours, and preferably 1 to 4 hours. It is important tonote that, for the purposes of the invention when the mixture is heatedto a temperature above the liquidus curve, B₂ O₃ and SiO₂ behave asimmiscible liquids at soak times and temperatures typically selected inpracticing the invention.

Cooling Step

When the mixture has been held at the selected elevated temperature forthe selected soak time, it then is cooled to ambient temperature. Thiscooling step may be performed continuously or in stages.

Particle Recovery

Residual B₂ O₃ contained in and on the SiO₂ particles is readily removedby washing with water. The compound B₂ O₃ generally is not consideredenvironmentally hazardous, and boron compounds may be additives incommercial compositions employing the SiO₂ particles. Thus, it may notbe necessary to take elaborate care to remove trace quantities of B₂ O₃contained in internal pores of the SiO₂ particles for many applications.

Particle Properties

Resulting SiO₂ particles have a substantially higher surface area andsubstantially lower bulk density than the particles introduced to theprocess due to internal porosity formed in the particles. Bulk densityof dry powder consisting of these particles typically is in the range of0.2 to 0.9 g/cc, compared to the bulk density of 1.0 g/cc for a typicalcommercial fused silica powder. Surface areas of the powders range from20 to over 400 m² /g. It should be recognized that physical propertiesof the products will depend on method of preparation includingcomposition of the mixture, temperature to which the mixture is heatedand soak time. Individual particles may have a "shredded wheat"appearance when viewed with a microscope. If the SiO₂ particles fed tothe process had angular morphology, the resulting particles will have amore regular morphology (i.e., less angularity).

Surprisingly, it also has been found that pellets pressed from theparticles possess unexpected properties when heated. For example, thosepellets pressed from silica particles produced by heating totemperatures in the range of about 450° C. to about the temperature atthe liquidus curve display a very low dimensional change, i. e., averagevolume change, of about ±0.5% when heated over the temperature range of100° C. to 900° C. The particles produced by heating to this temperaturerange typically have bulk densities in the range of 0.6 to 0.9 cc/g.

Those pellets pressed from silica particles produced by heating totemperatures above the liquidus curve possess the unexpected property ofhaving an Initial Negative Thermal Expansion Coefficient when heated.The term "Initial Negative Thermal Expansion Coefficient" as used hereinis defined as meaning that pellets consisting of the SiO₂ particlesactually shrink the first time the pellets are heated to a temperaturein the range of approximately 100° C. to 400° C., using the test methoddescribed in Example 1. This phenomenon may be observed over only aportion of the 100° C. to 400° C. range in order to satisfy thedefinition. The particles produced by heating to this temperature rangetypically have bulk densities in the range of 0.2 to 0.6 cc/g,preferably 0.3 to 0.5 cc/g.

Without being bound by theory, it appears that internal particle voidscontract (irreversibly) within at least a portion of the 100° C. to 400°C. temperature range, at a rate higher than thermal expansion of theSiO₂, causing a net reduction in pellet (and particle) size. FIGS. 2 and3 reflect this phenomena for pellets prepared from two representativelots of particles in accordance with the invention and are discussedfurther in the Examples.

While the above describes the particle properties being distinctdepending on the temperature to which the mixture of boron oxide andsilicon dioxide are heated, it should be recognized that depending ontemperature, soak time and the composition of the mixture, a continuumof properties can be achieved which encompasses the range of potentialvariable combinations.

INDUSTRIAL APPLICABILITY

The particles may be used in conventional applications for silicaparticles. Moreover, the properties discussed above make the particlesparticularly suited for specialty applications requiring an inexpensiveSiO₂ having low density, a porous structure, or more regular particlemorphology. For example, the relatively large surface area to volumeratios of these particles makes them potential candidates for use ascatalyst supports, adsorbents, thickeners, viscosity and/or rheologymodifiers and chromatography media. Also, the internal pores can beimpregnated with a liquid and the particles employed as a delivery orcontaimnent vehicle.

The high degree of internal porosity, which can trap air, also makes theparticles useful as a filler in specialty polymer insulationformulations. Entrapped air also lowers the dielectric constant of theparticles, making them useful as a filler for polymers used inelectronic applications.

The particles may be also particularly suited for use as a filler inpolymers employed in electronic packaging. The Initial Negative ThermalExpansion Coefficient can be used to help offset the typically highthermal expansion coefficients of organic resins used in electronicpackaging. Thus, the resin may be designed to have an overall thermalexpansion coefficient similar to that of the intended substrate orcomponent to be encapsulated. In this way stresses (e.g., "thermalmismatch") created during the fabrication can be minimized. Also, due tothe regular morphology of the particles, the particles are less likelythan conventional mechanical silicas to become lodged between connectorwires as the polymer composite is cast into place.

The particles may also be useful in numerous applications such as inadhesives, cosmetics and personal care products, greases, rubbers,coatings, sealants, pharmaceuticals, composites, foods, and inks.

Having described the invention, it now will be illustrated, but notlimited, by the following examples.

EXAMPLES Example 1

12 grams of silica powder, SiO₂ (Minsil 40, commercially available fromMinco Inc.), was dry mixed with 21.2 grams of boric acid, H₃ BO₃ (GRpowder from EM Science Industries Inc.) in an alumina crucible. A smallamount of deionized (DI) water was then added and stirred in to make amix with a paste-like consistency. This procedure was repeated again ina second alumina crucible. The two crucibles were then covered, placedin a small box furnace, and heated to 1000° C. and held at thattemperature for four hours. The temperature was then lowered to 500° C.and held there for thirty minutes followed by turning off the power tothe furnace and allowing the two crucibles to cool to room temperature.The crucibles were then immersed in about 3.5 liters of DI water in a 4liter glass beaker and manually scraped so as to remove the friable-likepowder pellet which had formed in each crucible. The powder/watermixture was then heated to about 90° C. while being agitated for onehour to solubilize the B₂ O₃ and isolate it from the SiO₂ powder. Themixture was allowed to cool to room temperature and then was reslurriedand allowed to set for 30 minutes. A major fraction of the powder,designated as A, was observed to have settled out of suspension while aminor fraction, designated as B, was observed to still be suspended inthe water. This B fraction was decanted off to another 4 liter beakerand allowed to settle overnight. The A powder was transferred to aBuchner funnel with filter paper and was washed with 8 liters of DIwater. After the B fraction had settled out of solution the water wasdecanted off and the powder was reslurried with another 4 liters of DIwater and isolated by transferring to a filter. Both powders were driedin a vacuum oven at 130° C. for 12 hours.

The weight of the A powder was 16.7 grams and the B powder was 6.3grams. The total yield of 23 grams agrees well with the starting weightof 24 grams of SiO₂ powder that was used. Three grams of the Minsil 40and A and B powders were each charged to a 10 cc graduated cylinder andmanually tapped 30 times to give approximate tap densities of 1.14,0.50, and 0.32 g/cc, respectively. X-ray diffraction data, surfaceareas, particle size distribution parameters d16/d50/d84/dmax asmeasured by laser light scattering, and mercury and nitrogen porosimetrydata were collected to characterize the three powders. The results areprovided in Table 1.

                  TABLE 1                                                         ______________________________________                                                       Minsil 40                                                                             A powder B powder                                      ______________________________________                                        X-ray diffraction                                                                              amorphous amor-    amor-                                         phous phous                                                                 d.sub.16 6 microns 7 6                                                        d.sub.50 25 15 11                                                             d.sub.84 57 31 23                                                             d.sub.max 148 105 105                                                         BET surface area 1 m.sup.2 /g 221 382                                         bulk density (Hg)* 100 g/cc 0.38 0.35                                         intrusion volume (Hg)* 0.46 cc/g 1.76 1.95                                    surface area of pores between 1.3 m.sup.2 /g 70.3 165.5                       17-3000 angstroms in diameter                                                 (N.sub.2)*                                                                    pore volume of pores between 0.0056 cc/g 0.0717 0.1644                        17-3000 angstroms in diameter                                                 (N.sub.2)*                                                                  ______________________________________                                         *(Hg) and (N.sub.2) represent data obtained from mercury and nitrogen         porosimetry, respectively.                                               

In addition to the above data the powders were characterized in terms oftheir thermal expansion behavior using a thermal mechanical analysis, orTMA. The data were collected using a Thermomechanical Analyzer, model#2940, made by TA Instruments. The powders were pressed into cylindricalpellets with approximate dimensions of 7 mm in diameter and 3 mm inheight at a force of 2,000 pounds. The pellets were then heated at about10° C./min up to 950° C. and the relative change in the height dimensionwas measured. These data are plotted in FIGS. 2, 3, and 4, where thex-axis is temperature and the y-axis is % dimension change. FIG. 4 isthe TMA data for the Minsil 40 powder. It shows a fairly typicalbehavior for a powder comprised of dense silica particles, expandingfrom about 100° C. to 600° C. and reaching a maximum expansion of about+0.5% at 600° C. The pellet made from the A fraction powder exhibits amarked contrast in its TMA data shown in FIG. 2. In this case, thepellet is observed to contract from about 100° C. to 400° C., yielding acontraction, or negative thermal expansion, of about -1.1% at 400° C.Upon further heating the pellet undergoes a linear expansion up to 950°C. The pellet made from the B fraction of powder also exhibits adifferent trend in its TMA data as shown in FIG. 3. In this case, thepellet is observed to exhibit a contraction from about 100° C. to about200° C. of about -0.4%. It then remains flat until about 300° C. atwhich time it exhibits an expansion up to about 500° C. that peaks atabout +0.25% at 500° C. A further increase in temperature results in acontraction of the pellet equal to -4.0% at 950° C.

These pellets were cooled to room temperature and then run again usingthe same temperature profile. The TMA curves for the second passes areillustrated with a dashed line. The character of these curves is quitedifferent from that of the original TMA data. In particular, FIG. 2shows a slight expansion up to about 0.3% upon reheating from 100° C. to500° C. This is very different from the initial heating of that pelletwhich exhibits a contraction of about -1.1% at 400° C. This implies thatthe observed negative thermal expansion is a one time phenomenon overthe temperature range of about 100° C. to 400° C., and is notreversible.

Comparative Example 2

The Initial Negative Thermal Expansion Coefficient exhibited by thepellet made from the A fraction powder of Example 1 is unexpected andclearly different from that of a pellet comprised of dense silicaparticles as shown in FIG. 4. Such behavior could be indicative of highsurface area powders and/or powders comprised of particles possessing ahigh degree of internal porosity. To determine if this is true threecommercially available powders were measured using TMA and the procedureas defined in Example 1. The silica powders used were:

a) Cab-o-sil(R), fumed amorphous silica, Grade PTG, made by CABOTCorporation and two different silica powders obtained from Alpha Aesar,a Johnson Mathey Company located in Ward Hill, Mass., defined as

b) an 8 mesh material, catalog #89346, with 1 cc/g of porosity, and

c) a 58 micron powder, catalog #89385, with 1 cc/g of porosity.

The TMA data for the pellets made from the Example 2-a, b, and c powdersare shown in FIGS. 5, 6 and 7, respectively. FIG. 5 shows a curvesimilar in character to that of the pellet made from the Minsil 40powder given in FIG. 4. FIGS. 6 and 7 are similar in character but quitedifferent from the trends exhibited in FIGS. 2 and 3. In these cases thepellets are observed to show approximately zero thermal expansion from100° C. to about 300° C. After 300° C. both pellets exhibit a negativethermal expansion up to 950° C., resulting in total negative thermalexpansions of approximately -5 and -6%, respectively, at thattemperature. Properties of the particles were measured using theprocedures of Example 1 and are provided in Table 2.

                  TABLE 2                                                         ______________________________________                                                      A Powder                                                                              B Powder C Powder                                       ______________________________________                                        BET surface area                                                                              204    m.sup.2 /g                                                                           259    374                                        bulk density (Hg)* 0.06 g/cc 0.65 0.37                                        intrusion volume (Hg)* 15.77 cc/g 0.95 1.45                                   surface area of pores between 195.8 m.sup.2 /g 382 552                        17-3000 angstroms in                                                          diameter (N.sub.2)*                                                           pore volume of pores between 0.55 cc/g 1.12 0.80                              17-3000 angstroms in                                                          diameter (N.sub.2)*                                                         ______________________________________                                         *(Hg) and (N.sub.2) represent data obtained from mercury and nitrogen         porosimetry, respectively.                                               

None of the pellets made from the powders of Example 2 exhibit aninitial negative thermal expansion over the temperature range of 100° C.to 400° C., as is the case for the pellet made using the A powderfraction from Example 1.

Example 3

A blend of 650 g silica powder (Minsil 40, commercially available fromMinco Inc.), and 621.6 g of boric acid (Boric Acid Technical Granularfrom U.S. Borax Inc.) were blended by shaking in a 1-gal plastic bag.The mixture of powders was charged to a 10 in.×10 in.×2 in. aluminasagger containing about 550 ml of deionized water. The mixture ofpowders and water was stirred to give a thin paste. The sagger wasplaced in a box furnace to about 500° C. and the temperature maintainedat about 500° C. for about 4 hrs. The sagger was allowed to cool slowlyin the closed furnace to about 85° C. The sagger was immersed in 7 to 8liters of hot tap water. This procedure aids in the removal of boronfrom the product silica as water soluble boric acid. The resultingslurry of silica in aqueous boric acid was heated at about 75° C. forabout 1.5 hrs. The slurry was filtered and the cake washed withdeionized water until the filtrate density was about equal to that ofthe deionized water. The wet cake was dried in an oven at about 120° C.for about 16 hrs to give about 540 g a white silica powder.

The properties of the product powder are shown in Table 3 and comparedto the Minsil 40.

The powder was also characterized in terms of its thermal expansionbehavior in a similar process to that described in Example 1. A sampleof the powder was pressed into a cylindrical pellet and then heated to atemperature of 950° C. and the relative change in the height dimension(volume) was measured. The data are plotted in FIG. 8. The pellet isobserved to change in dimension (volume) of a maximum of less than0.07%.

Example 4

The process of Example 3 was repeated to with a silica/boric acidmixture having a ratio of 25% B₂ O₃ and 75% SiO₂. The properties of theproduct powder are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                       Minsil 40                                                                             Example 3                                                                              Example 4                                     ______________________________________                                        X-ray diffraction                                                                              amorphous amor-    amor-                                         phous phous                                                                 d.sub.16 3.4 microns 5.1 5.2                                                  d.sub.50 17.5 16.4 19.2                                                       d.sub.84 47.4 38.7 43.8                                                       BET surface area 2.3 m.sup.2 /g 32 25                                         bulk density (Hg)* 0.99 g/cc 0.81 0.83                                        surface area of pores between 1.8 m.sup.2 /g 32.5 23.3                        17-3000 angstroms in diameter                                                 (N.sub.2)*                                                                    pore volume of pores between 0.01 cc/g 0.04 0.03                              17-3000 angstroms in diameter                                                 (N.sub.2)*                                                                  ______________________________________                                    

As can be seen from Table 3, the products of Examples 3 and 4 showincreases in surface area and pore volume, and decreases in bulk densityindicating a more porous product.

What is claimed is:
 1. A process for preparing silicon dioxide particlescomprising, in sequence:a. forming a mixture of dense naturallyoccurring or fused silicon dioxide particles having a surface area lessthan 5 m² /g and at least one boron compound selected from the groupconsisting of B₂ O₃ and boric acid; b. heating said mixture to atemperature in the range of 450° C. to the temperature at the liquiduscurve of said mixture for a time sufficient to alter the density and/ormorphology of said individual silicon dioxide particles; c. cooling themixture to ambient conditions; and d. recovering discrete silicondioxide particles, wherein the recovered silicon dioxide particles havean altered density and/or morphology as compared to the silicon dioxideparticles of step a.
 2. The process of claim 1 wherein the mixture ofstep a contains water and the process further includes a step ofremoving water from said mixture prior to heating of step b.
 3. Theprocess of claim 1 wherein the mixture heated in step b consistsessentially of 30 to 70% B₂ O₃ and 70 to 30% silicon dioxide particles,by weight.
 4. The process of claim 3 wherein the silicon dioxideparticles contained in said mixture of step a are crystalline.
 5. Theprocess of claim 3 wherein the silicon dioxide particles contained insaid mixture of step a are amorphous.
 6. The process of claim 1 or 3wherein the mixture is maintained at the temperature of step b for 1 to20 hours.
 7. The process of claim 1 wherein at least a portion ofresidual B₂ O₃ contained in or on the recovered SiO₂ particles isremoved by washing said particles with water.